VLA Discovers Giant Rings Around Galaxy Cluster

Astronomers using the National Science Foundation's
Very Large
Array (VLA) radio telescope have discovered giant, ring-like
structures around a cluster of galaxies. The discovery provides
tantalizing new information about how such galaxy clusters are
assembled, about magnetic fields in the vast spaces between
galaxy clusters, and possibly about the origin of cosmic rays.

Above, a combined radio/optical image shows
the galaxy cluster Abell 3376 in visible light
(blue) and radio (red) images. The giant radio
arcs surrounding the cluster were discovered
using the Very Large Array. The visible-light
image is from the Digitized Sky survey.

Below, an X-ray image of Abell 3376 made using
the European Space Agency's XMM-Newton telescope
shows a spectacular, bullet-shaped region of
X-rays coming from gas heated to 60 million
degrees Kelvin. The bullet shape results from the
supersonic collision of a smaller smaller galaxy
subcluster with the main body of the larger cluster.

Click on images for larger version.

Galaxy Cluster Abell 3376 (X-Ray)
CREDIT: Joydeep Bagchi, IUCAA, ESA

"These giant, radio-emitting rings probably are the result of
shock waves caused by violent collisions of smaller groups of
galaxies within the cluster," said Joydeep Bagchi, of the
Inter-University Centre for Astronomy and Astrophysics in Pune,
India, who led an international research team. The scientists
reported their findings in the November 3 edition of the
journal Science.

The newly-discovered ring segments, some 6 million
light-years
across, surround a galaxy cluster called Abell 3376, more than
600 million light-years from Earth. They were revealed because
fast-moving electrons emitted radio waves as they spiraled
around magnetic field lines in intergalactic space. "Even from
this large distance, the feeble radio waves were easily picked
up by the VLA, thanks to its very high sensitivity and unique
capability to make images of exceedingly faint radio-emitting
objects," Bagchi said.

The scientists also used the European Space Agency's XMM-Newton,
the world's most sensitive X-ray observatory, to observe this
extraordinary cluster of galaxies. "The advanced technical
capabilities of the orbiting XMM-Newton revealed a spectacular
bullet-like region of X-ray emission in this dynamically active
cluster," said Gastao B. Lima Neto, of the Institute of
Astronomy and Geophysics in Sao Paulo, Brazil, a co-author
of the research paper.

"Our X-ray observations strongly suggest a recent collision
and merger of two or more smaller clusters. Such a phenomenon is
among the most energetic events in the Universe after the
Big Bang.
Only a tiny fraction of the total energy of this collision, if
transferred to electrons, would cause them to emit the radio waves
observed by the VLA. However, the main question is, how this is
achieved," said Florence Durret of the Astrophysical Institute
of Paris, France, another of the researchers.

The scientists calculated that the total energy of the colliding
groups of galaxies would be enough to keep our Sun shining for
more than 20 sextillion years (2 followed by 22 zeros)!

"We think the shock waves that sped up these electrons came
from the collision of a smaller group of galaxies with the
main body of the larger cluster. When two such massive objects
crash into each other at supersonic speed, gigantic ripple-like
shock waves are created in the surrounding gas, which race out
to the outer regions of the forming cluster at a speed of
thousands of kilometers per second," Bagchi said.

"You can imagine that each cluster is like a supersonic aircraft,
moving faster than the speed of sound in the surrounding gas, and
just as you hear a sonic boom when shock waves from an airplane
pass by you, we believe that the situation in the Abell 3376 cluster
is similar, with ringlike radio structures tracing out the shock
waves," Bagchi explained.

Such a scenario also is supported by images of the cluster made
with the XMM-Newton and ROSAT X-ray satellites, as well as by
computer simulations, Bagchi added. The exact mechanism for
producing the shock waves is still open to question, the
scientists said.

"This is the first observational evidence for this type of
shock wave around a massive galaxy cluster," Bagchi said.
"This discovery will help us understand more about the thin
gas between the galaxies, and also about the magnetic fields
in the outskirts of such clusters -- magnetic fields whose
origin still is unknown," he said.

In addition, the scientists speculate that violent regions like
those in Abell 3376 may be sites from which
cosmic rays originate.
Cosmic rays are protons or atomic nuclei accelerated to nearly the
speed of light, and shocks such as those found in the collisions
of galaxy groups may be energetic enough to provide the required
amount of "kick."

"Some of the most energetic cosmic ray particles detected on Earth
may contain about 100 million times more energy than the highest energy
achieved so far in any man-made particle accelerator. Where do
these cosmic rays come from and exactly what process kicks them to such
stupendous energy is still a fascinating unsolved problem of physics,"
said graduate student Surajit Paul of the Institute for Theoretical Physics
and Astrophysisc at Wuerzburg University in Germany, who was on the
research team.

"A cosmic accelerator source containing powerful shock waves
and magnetic fields extending over millions of light years in
length is capable of accelerating a proton or nucleus to such
enormous energies. Although our observations do not conclusively
show the evidence for such particles, our VLA radio image does
show clearly that such structures are indeed present in this galaxy
cluster. Only future cosmic ray observations can tell if Abell
3376 is an ultra-high-energy cosmic ray source. We will continue
to explore this fascinating cosmic laboratory in the future,
employing some of the world's most sensitive radio, X-ray and
gamma-ray telescopes to reveal its mysteries," Bagchi said.